1. Field of the Invention
The present invention relates to a process of and an apparatus for thermally processing a crystalline thermoplastic resin film and is applicable to such industrial fields as packing foods and industrial products and lining bag-in-boxes and drums with the film.
2. Description of the Related Art
A plastic film is generally biaxially oriented by a tubular method and subsequently thermally processed to fix its molecular orientation or obtain stable dimensions thereof.
Under the process, when thermally processing a tubular film, for example Nylon - 6 film, in a flat shape by a tenter method at about 200 .degree. C., the upper and lower films melt and adhere to each other, so that the adhered films cannot be divided into two portions for use as products. This problem becomes formidable when using such a crystalline thermoplastic resin film as polyamidies.
A modified tenter method (Japanese Patent Publication No. 46-15439) has already been conceived to overcome this problem, in which a biaxially oriented film in flat shape is cut out to be divided into two films at its lug portions, the cut films are conveyed by an endless conveyer to a tenter device with a certain space therebetween, and the films are still thermally processed while both side portions thereof are clipped in the tenter device (Japanese Patent Publication No. 46-15439).
Additionally, it is also possible to employ another method to avoid the aforementioned problem. The other method is a tubular method in which a tubular film is thermally processed while pressurized air is fed thereinto.
According to the modified tenter method, the two films do not adhere to each other, but a so-called bowing appearance is produced (a middle portion of a film is delayed in being elongated in its moving direction compared with its side portions while hardening), whereby there results some sags which may cause some harm to the anisotropy of and secondary processing (e.g., printing) of the film. It is of course recognized that if the temperature for thermally processing through the method is controlled to be at a low level, the bowing appearance is suppressed. However, when thermally processing by boiling, retorting and the like, there may be a problem owing to a high contraction ratio of the film.
According to the tubular method, there is a problem that if the thermal process is executed over 180.degree. C. to achieve a high dimensional stability, the film bubble may flare to thereby make it difficult to maintain a constant process. If the method is done at a low temperature to avoid this problem, high dimensional stability cannot be attained.
The applicant has described a method of and an apparatus for thermally processing, within two steps, a crystalline thermoplastic resin film which is biaxially oriented by a tubular method in Japanese Patent Application No 63-296575. In this application, the second thermal process is limited to the use of a tenter method, but the first thermal process is not limited in detail.
An object of the present invention is to provide a method of and an apparatus for thermally processing a crystalline thermoplastic resin film which avoids the melting and sticking of the film and diminishes the bowing ratio thereof.